Monitor soybeans for green stem/bean syndrome

Green stem syndrome (labeled as a disease by some) in soybeans occurs when stems stay green after the pods mature. Leaves and petioles may remain on the plant. Conditions associated with this syndrome can result in delayed harvest, reduced harvest speed, and contamination of harvested seed with green plant material.

Green stem syndrome has been attributed to fungal and viral diseases, insects (mainly stink bugs), and environmental stresses (mainly drought). All of these maladies typically result in a reduced number of pods and alter the source-sink ratio in favor of the source or vegetative tissue.

A recent article in Crop Management, an online journal available by subscription through the American Society of Agronomy, reports on finding the cause of green stem syndrome. The article, written by Dennis Egli and William Bruening of the University of Kentucky, is a summarization of results from experiments conducted for two years at Lexington, Ky.

Experimental plots were irrigated to minimize drought stress. Treatments of 25 percent and 50 percent pod removal were applied at beginning seedfill to minimize additional pod set after pods were removed.

The delay in time to brown stems was dramatic with depodding. The delay for MG III and MG IV varieties averaged greater than two weeks in the 25 percent depodded treatment and greater than four weeks in the 50 percent depodded treatment. Stem maturation was not complete in some treatments when frost occurred 30 to 50 days after the controls were mature.

Green stems resulting from depodding were associated with higher stem concentrations of sugars, starch, and nitrogen. Starch and nitrogen are normally translocated from stems to seeds during seed filling, and this movement is limited with a reduction in the number of seeds.

Their results indicate that green stem syndrome in soybeans is caused by a reduction in number of pods. This may provide an explanation for the many proposed causes of this problem. Any agent (disease, insect, environmental stress) may result in green stem syndrome indirectly by reducing the number of pods.

Considerable varietal variation in green stem syndrome occurred in their study. This may occur in a producer environment in response to the cause of pod reduction. Some varieties may be more susceptible to a particular pathogen or insect, and thus suffer more pod loss as a result of the infestation by the causal agent. This difference in susceptibly of varieties to damage from the causal agent should result in a different level of depodding, and thus a different level of green stem occurrence.

Importantly, delays in the development of mature pods or seeds drying down on depodded plants were much smaller (usually less than seven days) than the delays in stem maturity. This indicates that pod and seed maturation do not require maturation of the vegetative plant, and that green stem and green bean syndromes in soybeans may be unrelated.

Green bean syndrome is associated with the seed not drying down. It is likely that seed staying green on mature soybean plants is related to direct damage to the seed by an agent such as stink bugs. Thus, insects that infest plants can cause green stem syndrome by reducing the number of pods, and also can cause green bean syndrome by inflicting physical damage to seed in the pods.

The take-home message is this:

Some pests of soybeans, both disease and insect, can cause green stem syndrome by causing pod loss.

Some pests, mainly insects and especially stink bugs, can cause both green stem and green bean syndrome by causing both pod loss and damage to immature seed.